I imagine you faced a variety of obstacles when adding support for dynamic imaging to a device that only performed static imaging. What was the development of the core sensor like?
- Watanabe
- Admittedly, there were several obstacles. We had particular trouble designing a sensor with a high level of sensitivity.
Are dynamic images possible using the same sensor sensitivity as static images?
- Watanabe
- It is possible if the same X-ray strength used for static imaging is used for a prolonged period. However, this increases the radiation exposure for the patient undergoing the examination. To address this, we needed to significantly lower the amount of X-ray radiation per unit of time and improve the sensor sensitivity to enable signals to be received at up to 30 frames per second.
If the X-rays emitted are weak, the light emitted from the phosphor layer on the device is also weakened, isn't it?
- Watanabe
- That's right. Because of this, the number of electrons generated by the optical sensor was also reduced. Furthermore, since it's difficult to transfer a small number of electrons to the monitor using a switching element (TFT), noise is increased. The problem thus becomes two-fold.
How did you resolve this?
- Watanabe
- We increased the area of the optical sensor to enable sufficient absorption of visible light converted on the phosphor layer, and increased the amount of light received for each pixel made up of an optical sensor and switching element (TFT). We also lowered the resistance of the wiring within each pixel to increase the transfer speed.
What? That sounds impossible? The surface area of a single pixel is the same as in the past isn't it?
- Watanabe
- Yes. The size of the pixels on the imaging device hasn't changed. Previously, the wiring was adjacent to the optical sensor, but we changed this to a two-layer structure. Large optical sensors were placed below the phosphor layer and the wiring was placed below these. This made it possible to freely contain the optical sensor and wiring within a single pixel.
To support dynamic images, you needed to operate the redesigned sensors at high speed. What innovations did you adopt in the elements themselves with regard to this point?
- Watanabe
- In order to operate the sensor at high speed, various wiring voltages need to be kept within a certain range throughout the entire light receptor surface. To achieve this, we needed to lower the resistance and capacity of the wiring, and so we conducted numerous simulations in our search for an answer. This design method was essential during the development of a sensor for dynamic-imaging use.
I see. It`s a very advanced design. Was it difficult to manufacture the system according to the design?
- Watanabe
- We had problems, such as the film peeling and wire disconnection. The actual manufacturing process was as difficult as, or perhaps even more difficult than, the design.
Did you proceed through trial and error?
- Watanabe
- Unforeseen problems often arise when making new elements with such a delicate structure. In addition to problems that can be identified by enlarging each element to check them visually, there are also problems that cannot be seen, and the only option in such cases is to think about what is causing the problem while trying a variety of tests. Not only that, we often thought we had succeeded, only to be disappointed at the discovery of irregularities and weird patterns once we had loaded the image (laughs).
